Usually 10 seconds won't hurt as long as you give it time to cool down, in the winter it would be less time than in the summer, on my b6200 I've gone for 20 seconds but gave it two minutes or so before doing it again, that's usually main reason for starter failure, it overheating and continuing to run the starter, I'm sure many other people will have their own opinion on that but 5 seconds is ideal, 10 is a bit much, 15 is pushing it, and 20 like I've done is borderline stupid on any vehicle let alone a diesel, but it was the original starter from 1989, in its later years the starter had to be replaced anyway and it always had winter starting problems, never really made any noticeable difference but I really don't recommend doing that, 10 won't hurt it, but let it cool down if it doesn't start in that time, and like Hokie said I'd check other things and use a block heater, it's worth not having the headaches if you need to plow or snowblower. My 6200 issues were the glow plugs, the compression lever getting stuck ever so slightly open, and the dynamo not charging anymore due to the previous owner, which also led to the glow plug failure and going through batteries every two years